1. Field of the Invention
The present invention relates to an improved radiant vessel passive cooling system for modular liquid-metal cooled pool-type nuclear reactors such as the type disclosed in U.S. Pat. No. 4,508,677 to Craig et al.
In the event of an emergency during the operation of a sodium or sodium-potassium cooled nuclear power plant, it is sometimes necessary to shut down the fission reaction of the reactor core. To accomplish the "shutdown," the control rods are fully inserted between the fuel assembly rods in the reactor core to absorb neutrons. However, a significant amount of heat continues to be produced by the fission products. It is necessary that the structures surrounding the reactor be capable of dissipating such residual heat without incurring any structural damage. The heat capacity of the coolant and the overall structure aids in dissipating the residual heat. The materials from which the surrounding structures are constructed may not be able to safely withstand high temperatures. For instance, thick concrete walls typically are used as part of the containment housing for a nuclear reactor. However, concrete cannot be relied upon to withstand high temperatures since concrete begins to splay and crack under high temperature conditions. An auxiliary cooling system to safely remove the heat from the reactor structure during a shutdown is necessary.
2. Description of the Related Art
Conventional nuclear reactors have utilized a variety of elaborate energy driven cooling systems to dissipate heat from the reactor. In many of the situations warranting a shutdown, the energy supply to the cooling systems makes the cooling systems themselves subject to failure. For example, pumps and ventilation systems to cool the core may fail. Furthermore, if operator intervention is necessary, there are foreseeable scenarios in which the operator would be unable to provide the appropriate action. The most reliable and desirable cooling system would be a completely passive system which could continuously remove the residual heat generated after shutdown.
For modular liquid-metal cooled reactors such as the type disclosed in U.S. Pat. No. 4,508,677, which produces on the order of 200-500 Megawatts (thermal), utilizing sodium or sodium-potassium as the coolant provides numerous advantages. Water cooled reactors operate at or near the boiling point of water. Any significant rise in temperature results in the generation of steam and increased pressure. By contrast, sodium or sodium-potassium has an extremely high boiling point, in the range of 1800 degrees Fahrenheit at one atmosphere pressure. The normal operating temperature of the reactor is in the range of about 900 degrees Fahrenheit. Because of the high boiling point of the liquid metal, the pressure problems associated with water cooled reactors and the steam generated thereby are eliminated. The heat capacity of the liquid metal permits the sodium or sodium-potassium to be heated several hundred degrees Fahrenheit without danger of materials failure in the reactor.
The reactor vessels for pool-type liquid-metal cooled reactors are essentially smooth, sealed cups without any perforations to interrupt the integrity of the vessel walls. This sealing is essential to prevent the leakage of liquid metal from the primary vessel. The vessel surfaces must also be accessible for the rigorous inspections required by safety considerations.
In the typical sodium cooled reactor, two levels of sodium loops are used. Usually, a single primary loop and two or more secondary loops are used. The primary loop contains very radioactive sodium which is heated by the fuel rods. The primary loop passes through heat exchangers to exchange the heat with one of the nonradioactive secondary sodium loops. In general, sodium cooled reactors are designed to incorporate redundant secondary loops in the event of failure of one loop.
Upon shutdown of the reactor by fully inserting the control rods, residual heat continues to be produced and dissipated according to the heat capacity of the plant. Assuming that the reactor has been at full power for a long period of time, during the first hour following shutdown, an average of about 2% of full power continues to be generated. The residual heat produced continues to decay with time.